The resonant coupling of atoms or spins (electronic or nuclear) to cavity modes has been a mainstay area of research in the field of quantum optics for many decades, and has recently attracted renewed interest in the field of quantum information processing and quantum computing where the spins serve as a quantum memory (qubits). However, the individual coupling of magnetic spins to the electromagnetic field of a cavity mode is too weak to allow for efficient coherent transfer of excitation (information) to and from the spins on a timescale shorter than the spin decoherence and cavity decay times. This requires the spin-cavity system to be in the “strong coupling” regime, where the coupling rate exceeds both the spin decoherence and cavity decay rates.
The spin-photon coupling rate of a single electronic spin is given by gs=γ√{square root over (μ0ℏωc/2Vm)}, where γ is the electron gyromagnetic ratio, μ0 is the vacuum magnetic permeability, ωc/2π is the resonant frequency and Vm is the magnetic mode volume of the cavity mode. The single spin-photon coupling is in the millihertz range for cavity frequencies in the 1-10 GHz range. It can be increased by reducing the mode volume (physical size of the resonator), but practically it is difficult to achieve a spin-photon coupling rate greater than 1 Hz. This upper limit is anyway impractical for spin-cavity excitation transference considering that spin coherence times are usually of the order of 10-1000 μs even at low temperatures. If a large number of spins N are placed within the cavity mode and close enough together so that they experience the same coupling, then through their individual interaction with the cavity mode they behave collectively as a so-called “spin-ensemble” with a spin-photon coupling enhanced by a factor √{square root over (N)}. For a collection of 1014 spins, the ensemble spin-photon coupling rate is enhanced by 7 orders of magnitude to the MHz range. This means that the coherent transfer of excitations between the spin-ensemble and the cavity mode occurs on the 1 μs timescale, which if less than the spin-decoherence and cavity-decay times ensures that the strong-coupling regime has been reached. This coherent exchange of energy back and forth between a spin-ensemble and the cavity mode produces quantum oscillations known as Rabi oscillations, which are the hallmark of a system in the “strong coupling” regime which permits spin-operations to be performed on a “qubit”.
These phenomena are usually observed at cryogenic temperatures with spin-ensembles consisting of a few million spins (in the milli-Kelvin range). Cryogenic temperatures are required for a number of reasons: in order to reduce the influence of thermal photons, to mitigate decoherence effects, and also to provide thermal polarization of the two-level spin system.
In more detail, existing methods of establishing coherent quantum oscillations between spin-ensembles and cavities rely on refrigeration to cryogenic temperatures in order to satisfy a number of criteria:                (i) Currently, diamond-NV centres are a popular means of providing a population of triplet states that can behave as an ensemble of spins. Spin polarization may be achieved due the preferential population of the lower states at low temperatures according to the Boltzmann distribution. Polarization through optical pumping and spin-selective intersystem crossing is also possible. NV-diamond samples usually have NV concentrations of 1 ppm and 10 ppm at most.        (ii) In order to increase the spin-photon coupling, small mode volume resonators are realised using superconducting niobium stripline resonators. These require cooling below a few Kelvin in temperature and only provide quality factors of a few thousand.        (iii) The low concentration of NV centres and the small sample volumes limit the available number of spins to be in the region of 106-107. [1]        
Some researchers have reported using transition metal oxides (such as YAG) to provide a greater concentration of spins [2]. However, this approach still relies on cryogenic temperatures in order to preferentially populate and thus polarize the spin sub-levels.
It will be appreciated that the use of cryogenic temperatures significantly increases the complexity, usability and cost of practical systems for establishing coherent quantum oscillations, in turn hindering the development of quantum information processing systems. There is therefore a desire to achieve quantum oscillations at room temperature (i.e. around 293 K).
Further background art is provided in WO 2013/175235 A1, which discloses a device and method for generating stimulated emission of microwave or radio frequency radiation, e.g. to produce masing. Additional background art in respect of masing is provided in WO 2014/027205 A2.